Applicant""s invention relates generally to dry-type transformers having a ferromagnetic core, a high voltage winding, and a low voltage winding, and more particularly, to a method of reducing audible sound radiating from the transformer.
A transformer generally consists of a laminated, ferromagnetic core, high voltage windings, and low voltage windings. It is well known that transformers emit a certain level of audible noise. If a transformer is located outdoors, the noise level can be considerably higher than one that is located indoors. A power transformer located in a school or office building could be quite distracfful if it gave off excessive noise. A majority of the noise is caused by magnetostriction of the core laminations when the transformer is energized. The elastic deformation of the core that accompanies this energization occurs at a rate twice the line frequency. These deformations cause the individual core laminations to vibrate as they change shape due to the elastic deformation. This causes air columns to be formed in the spaces between the core and the windings and other adjacent parts of the transformer and its enclosure, if one is present. These air columns will cause audible sound as they move between the various parts of the transformer. The sound level is affected by the line frequency, the ambient sound level, and the surrounding environment.
Controlling the noise level of indoor installations can become quite expensive if it is done at the installation itself. For example, a separate sound-proofed room could be built around the installation. This may not be practical and it would be a better solution to control noise within the transformer itself. Sound is transmitted from the transformer to the rest of the installation due to vibration if the transformer is in direct contact with solid structural elements such as the floor or walls. Radiated sound through air may impinge on surrounding walls, causing them to vibrate and transmit sound on the other side of the wall. Sound can also travel through conduit and other electrical connecting means, and through heating and ventilating equipment. Various methods have been implemented as standard manufacturing and engineering practices to reduce noise levels. These methods include manufacturing controls of the core laminations to reduce the effects of magnetostriction by maximizing flatness and minimizing stresses of the laminations and insuring good core joints. The laminations may be cemented or coated. High silicon content steel could be used as the core material. Reducing the induction of the transformer will also reduce the effects of magnetostriction. This, however, increases the size, weight, and cost of the transformer.
It has been understood that noise emission is dependent primarily on the power rating of the transformer and the flux density of its core. Transformer loading has, until recently, contributed little to the overall noise level. However, as the above improvements have been made to the core material, noise emitted from the load dependent windings becomes more of a factor. Load noise, the additional noise emitted above the no-load level, is caused by the electromagnetic forces that result from leakage fields surrounding the transformer that cause vibrations in the windings, shields, or the enclosure housing the transformer. Methods have been developed to reduce the noise levels caused these forces. These methods have been more adaptable to oil-filled transformers or those that are surrounded by a tank. By using thicker or double walled tanks, some of the generated noise can be self-contained. This has been found to be least effective and costly. Core vibration isolators can be used to interrupt the noise path between the transformer and the structural elements. In the case of an oil filled transformer, complicated designs of an effective restraining method are required to prevent damage to the transformer during shipping. Resilient absorbers have been applied to the interior surfaces of the tank walls so that the absorbers compress instead of the tank walls, thus reducing the vibrations on the walls caused by magnetostriction. These absorbers can be applied as a type of lining to the interior surfaces of the tank walls. The resilient lining is softer than the tank wall so that they can compress instead of the wall. Another method, as disclosed in U.S. Pat. No. 3,579,164, uses flexible fibers that have one end coupled to the core and to the tank walls. The loose ends are allowed to hang free so that they can absorb the energy from the air columns generated by the magnetostriction forces.
All of these methods have varying degrees of effectiveness, and are generally more applicable to liquid or gas filled transformers. It would be desirable to develop a system or method to reduce noise levels in dry-type power transformers that is more cost effective and is readily adaptable to present transformer design without having to modify the transformer enclosure.
Accordingly, the principal object of the present invention is to provide a transformer with a high voltage winding and a low voltage winding surrounding a core with reduced emitted noise which overcomes the above mentioned disadvantages.
A further objective of the invention is to provide a method for manufacturing a transformer with reduced emitted noise levels without modifying an enclosure which houses the transformer.
In one embodiment of the invention, the inner or low voltage coil is constructed using a VPI resin encapsulated process. The outer coil or high voltage coil is a cast resin coil and is also fabricated using a VPI process, with the chief difference being that the resin is poured into a mold containing the coil, allowing the curing to take place inside the mold. The transformer is then assembled by inserting the inner coil over an iron laminated core and then inserting the outer coil around the inner coil. Sound damping pads are installed in the air gap between low voltage coil and the iron core. They are placed in alternating patterns at the top and the bottom of the low voltage coil. The pads are constructed of a suitable material that has sound absorbing properties. The alternate pattern of the pads serve as an air baffle to the air column that is generated by the magnetostriction. The resultant assembly is then secured with appropriate clamps and mounting feet, along with terminal means for external connections.
Other features and advantages of the invention will be apparent from the following specification taken in conjunction with the accompanying drawings in which there is shown a preferred embodiment of the invention. Reference is made to the claims for interpreting the full scope of the invention which is not necessarily represented by such embodiment.